David Marshall

Abstract:

The global overturning circulation (GOC) is the largest scale component of the ocean circulation, associated with a global redistribution of key tracers such as heat and carbon. The GOC generates decadal to millennial climate variability, and will determine much of the long-term response to anthropogenic climate perturbations. This review aims at providing an overview of the main controls of the GOC. By controls, we mean processes affecting the overturning structure and variability. We distinguish three main controls: mechanical mixing, convection, and wind pumping. Geography provides an additional control on geological timescales. An important emphasis of this review is to present how the different controls interact with each other to produce an overturning flow, making this review relevant to the study of past, present and future climates as well as to exoplanets’ oceans.

Abstract:

A parameterization for barotropic eddy potential vorticity (PV) fluxes is introduced which applies both an energetic and an enstrophetic constraint to a down-gradient PV mixing closure. An eddy kinetic energy budget and an eddy potential enstrophy budget are employed to constrain the parameterized eddy PV fluxes. Through the budgets, the parameterization facilitates a bidirectional exchange of kinetic energy between the parameterized eddies and the large-scale flow, and a conversion of potential enstrophy from the large-scale flow to the parameterized eddies. The parameterization is tested in simulations of barotropic, freely-decaying turbulence in a doubly periodic domain over variable bottom topography. The simulations show that employing the parameterization results in an upscale transfer of kinetic energy emerges on average, consistent with quasigeostrophic theory. Furthermore, the kinetic energy and potential enstrophy budgets employed are sufficient to constrain the large-scale flow in a realistic manner when compared to an eddy-resolving model. As a result, a topography-following flow of the correct magnitude emerges in a coarse-resolution model with parameterized eddy effects. Dissipation in the coarse-resolution simulations is significant, leading to the most significant source of discrepancy between the coarse resolution simulation with parameterized eddy effects and the eddy-resolving simulation. This work constitutes a first step towards the ultimate aim of parameterizing both baroclinic and barotropic turbulence. How this may be achieved by integrating this parameterization with other methods in more realistic ocean simulations is discussed.

Abstract:

The reduced sensitivity of mean Southern Ocean zonal transport with respect to surface wind stress magnitude changes, known as eddy saturation, is studied in an idealized analytical model. The model is based on the assumption of a balance between surface wind stress forcing and bottom dissipation in the planetary geostrophic limit, coupled to the GEOMETRIC form of the Gent–McWilliams eddy parameterization. The assumption of a linear stratification, together with an equation for the parameterized domain integrated total eddy energy, enables the formulation of a two component dynamical system, which reduces to the non‐linear oscillator of Ambaum and Novak (2014, https://doi.org/10.1002/qj.2352) in a Hamiltonian limit. The model suggests an intrinsic oscillatory time scale for the Southern Ocean, associated with a combination of mean shear erosion by eddies and eddy energy generation by the mean shear. For Southern Ocean parameters the model suggests that perturbing the system via stochastic wind forcing may lead to relatively large excursions in eddy energy.

Abstract:

The mesoscale spectrum describes the distribution of kinetic energy in the Earth's atmosphere between length scales of 10 and 400 km. Since the first observations, the origins of this spectrum have been controversial. At synoptic scales, the spectrum follows a −3 spectral slope, consistent with two-dimensional turbulence theory, but a shallower −5/3 slope was observed at the shorter mesoscales. The cause of the shallower slope remains obscure, illustrating our lack of understanding. Through a novel coarse-graining methodology, we are able to present a spatio-temporal climatology of the spectral slope. We find convection and orography have a shallowing effect and can quantify this using “conditioned spectra.” These are typical spectra for a meteorological condition, obtained by aggregating spectra where the condition holds. This allows the investigation of new relationships, such as that between energy flux and spectral slope. Potential future applications of our methodology include predictability research and model validation.

Abstract:

A large part of the variability in the Atlantic Meridional Overturning Circulation (AMOC) and thus uncertainty in its estimates on interannual timescales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (< 2 days) and synoptic (2-10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current.